In an electric power steering system for applying a proper steering assist force to reduce the steering force of the driver, a three-phase brushless DC motor is used in more cases in place of the conventional motors with brushes. A three-phase brushless DC motor is employed for reasons such as a simple mechanical structure and excellent controllability.
A control apparatus for a brushless motor is required to have a function of detecting a failure occurring in a CPU included in the control apparatus. In accordance with a method disclosed in U.S. Pat. No. 6,373,217 (JP2001-018819A), a sub-CPU is used to control a main CPU for controlling the brushless motor.
In another method disclosed in U.S. Pat. No. 6,513,619 (JP 2002-67985A), no sub-CPU is used to monitor a failure of a CPU for controlling a DC motor. With this method, however, the direction of the rotation of the motor needs to be detected. In order to solve this problem, a circuit for determining the drive direction of the motor has been proposed. As described in US 2003-0151383A1 (JP2003-235285A), a drive-direction determination circuit determines the drive direction of the brushless motor based on an electric angle of the motor. In turn, the electric angle of the brushless motor is found based on a signal output by a resolver using a region determination circuit.
In U.S. Pat. No. 6,373,217, a control system needs to be constructed with a plurality of CPUs, requiring a complicated configuration of a circuit composing the control system and a large size of a control apparatus accommodating the circuit. In addition, as a premise, the methods disclosed in U.S. Pat. No. 6,513,619 and US 2003-0151383A1 assume that the number of poles employed in the brushless motor is equal to the number of poles employed in a resolver for detecting the rotational position of the motor. In this case, one period of an electric angle Ea (or resolver rotation angle) of 360 degrees is divided into eight regions, i. e., regions A0 to A7, each having an angle of 45 degrees as shown in FIG. 10A. When the electric angle exists in region R1 or R5, for example, the rotational direction of the brushless motor can be determined based on a relation between the magnitudes of U-phase and V-phase currents (Iu and Iv) flowing through the motor.
In some cases, however, the number of poles employed in the brushless motor may not be equal to the number of poles employed in the resolver for detecting the rotational position of the motor. It is here assumed that the number of poles employed in the brushless motor is 14 while the number of poles in the resolver is two. In this case, in one period of the electric angle of the resolver, seven periods of each phase current of the brushless motor exist as shown in FIG. 10B. Thus, when the electric angle exists in region A1 or A5, the magnitude relation between the U-phase and V-phase currents of the motor is indeterminate, so that the rotational direction of the brushless motor cannot be determined. As a result, the operation to monitor a failure of the CPU cannot be carried out.